1. Technical Field
The present invention relates to an optical device and an optical modulation apparatus.
2. Related Art
A Mach-Zehnder optical device is known that uses a lead lanthanum titanium zirconium oxide (a PbLaZrTiO-type complex oxide, shortened hereinafter to “PLZT”) crystal, in which the change in refractive index with respect to applied electric field strength differs from that of LiNbO3 (LN), as shown in Patent Document 1, for example. Such an optical device applies a bias voltage and a control voltage to two electrodes outside the Mach-Zehnder waveguide, to function as an optical modulator, as shown in Non-Patent Document 1, for example.    Patent Document 1: Japanese Patent Application Publication No. 2006-58837    Non-Patent Document 1: G. H. Jin, et al., “PLZT Film Waveguide Mach-Zehnder Electrooptic Modulator”, Journal of Lightwave Technology, Vol. 18, No. 6, June 2000
This optical device applies a high-speed control signal to the two electrodes outside the Mach-Zehnder waveguide. In this case, it is necessary to provide a plurality of bias tees, additional circuits, and differential signal drivers, for example, resulting in a complicated control circuit. Furthermore, in this case, the electrode within the Mach-Zehnder optical waveguide functions as a GND, and therefore there are problems involved in designing the electrode such as, when a high-frequency electric field is applied, the surface area of the electrode is insufficient for achieving the GND function.